The outflow vessels of the chick heart, pulmonary arteries, aortic and right and left brachiocephalic vessels are derived from the six paired aortic arches which course from the anterior aortic sac to the paired dorsal aortae. Experimental evidence suggests that blood flow through the aortic arches is a mechanism for determining which arch will involute and which will remain patent. Information derived from measurements of human congenital aortic arch anomalies also implicates a hemodynamic mechanism. This proposed research will idenfity the normal aortic arch blood flow patterns and the effect of alteration of these patterns on the morphogenesis of the aortic arch system in the chick, and in a mammalian system. The results of these experiments will provide further information concerning the role of blood flow in the pathogenesis of human congenital cardiac abnormalities of the aortic arch.